Analog video techniques have existed for many years whereby a live action video signal may be composited against a background of another arbitrary image when the foreground subject is set in front of a colored screen (typically a green or blue screen). For example, a local news weatherman may be composited against a background of a weathermap when the weatherman is actually standing in front of a blue or green colored screen. In some instances, a table of red, blue and green spectra is produced, and the background color range is interactively selected from the table. The composited image depicts the weatherman in front of the weathermap while the blue or green background is generally eliminated. Such compositing techniques are often referred to as "chroma key" techniques.
More recently, techniques have been developed using digital hardware whereby a digitized image may be stored in a framestore or in a memory comprised of pixels, where each pixel consists of three values corresponding to the red, green, and blue intensities at the appropriate pixel location on the video output device when the full color image is displayed. Such techniques, however, have several drawbacks. One drawback is that the screen in front of which the subject is shot must usually be either pure green or pure blue due to the nature of the chroma-key process. A second drawback is that the screen in front of which the subject is shot must be evenly lit. Uneven lighting causes uneven shading of the screen, making it difficult for such systems to distinguish between subject and screen.
Another drawback with conventional techniques is that no part of the background color may appear in the foreground subject. For example, if the background screen is blue, then no part of the subject may be blue or else the blue part of the subject may disappear or become translucent in the final composite. The same is true with a green screen and green parts of the subject.
More refined techniques have been developed to allow the subject to contain colors of similar hue as the background screen in front of which the subject was shot (albeit darker or lighter so as not to be confused with the screen itself), but these techniques usually degrade the quality of the subject colors in the final composite. Further, such techniques may not provide proper treatment of the fringe regions of the composited subject (except under strict lighting conditions), thus inadequately removing the "seam" of the original screen color surrounding the subject.
Means presently exist to touch-up extracted and treated digital images in order to correct discrepancies between the original subject image and the treated image. A "stencil brush" (i.e., a predefined graphical area on a computer screen controlled by a user) may be dragged over the fringe region of the treated image which will add the original foreground color to user-selected areas of the image. An "inverted stencil brush" similar to a stencil brush but has the effect of erasing the foreground color in user-selected areas. Neither of these touch-up methods is fully satisfactory, however, because they do not treat the translucent area of the seam which may contain remnants of the original screen color.
It would therefore be advantageous to have a means for compositing against an arbitrary background image a digitally acquired image of a foreground subject shot in front of a colored screen. It would further be advantageous to provide such a means without having restrictions placed on the lighting of the subject, so that the subject may be lit in a manner which matches the lighting conditions that originally existed or which exist artificially in the background image. It would further be advantageous to have a means for extracting a foreground subject which contains colors that are close to the color of the background region, and without limiting the background region to be a primary color but while allowing some gradations of hue in the background region. It would further be advantageous to have a means for treating a digitally acquired subject image such that, regardless of how the subject was lit, colored borders around the seam or fringe region of a subject are minimized or eliminated. It would further be advantageous to have a fast and efficient means for touching up the subject borders in those cases where the colors of the subject border are very close to the color or colors of the surrounding screen.